I have another cartridge case that I collected at a WWII site on Pohnpei (formerly Ponape) Island in Micronesia during my archaeological survey last year.
It is a broken cartridge case which measures 20.5 mm in diameter of the rim. The cartridge bore the headstamp: “U” at 12 o’clock, “2” at 4 o’clock, and “4” at 8 o’clock. (I like to post a photo, but have a problem again…)
My friend identified wrote me that it was for a Japanese Navy 13 mm machinegun. According to an US intelligence report (Bureau of Ordnance, 1946. Japanese Explosive Ordnance. Ordnance Pamphlet 1667. Washington, D.C.: Bureau of Ordnance, Navy Department.), it was probably used for “Type 93 AAMG” or “Type 3 fixed/flexible ACMG”.
Please let me know any information on this cartridge case (e.g., headstamp, gun type). Thank you for your information.